1. Field of the Invention
In accordance with the present invention, germanium is used to enhance the performance of silicon-based semiconductor devices. More specifically, germanium is used to create a germanium-rich layer at the silicon surface to provide lower contact resistance in the source and drain regions of MOS transistors.
2. Discussion of the Prior Art
The inherent properties of germanium that are capitalized upon in accordance with the present invention are as follows. First, it is well known that at normal integrated circuit operating temperatures (approx. 27.degree. C.), the drift mobility of electrons and holes in germanium exceeds the drift mobility in silicon by a factor of from 3-10. As discussed by S. N. Sze, "Physics of Semiconductor Devices", John Wiley & Sons, New York, 1981, Second Edition, page 29, the actual drift mobility factor is a function of impurity concentration. Second, the diffusion of silicon through germanium is faster than the diffusion of germanium through silicon. Third, the intrinsic resistivity of germanium (47.OMEGA..cm) is four orders of magnitude smaller than the intrinsic resistivity of silicon. Fourth, as discussed by A. R. Srivatsa et al, "Nature of Interfaces and Oxidation Processes In Germanium Implanted Silicon," Journal of Applied Physics, 65:4028 (1989), the wet oxidation of silicon with a germanium implant present can be used to form epitaxial layers of germanium. As disclosed by D. Fathy et al, "Formation of Epitaxial Layers of Germanium on Silicon Substrates by Germanium Implantation and Oxidation", Applied Physics Letter 51:1337 (1987), these germanium epitaxial layers can be formed without forming germanium oxide layers.
Pfiester et al., U.S. Pat. No. 4,835,112, teaches that co-implantation of germanium with phosphorous or boron will retard diffusion of the electrically active dopant species during the fabrication of semiconductor devices. Holland et al., U.S. Pat. No. 4,920,076, teaches that conversion of silicon to silicon-dioxide is enhanced by the prior implantation of germanium, and that the result of such oxidation will be to concentrate the germanium in the vicinity of the silicon silicon-dioxide interface. However, combining these two teachings leads to a technology which will produce extremely shallow junctions owing to the electrical interactions between the implanted germanium and the active dopant species, which will cause the dopant species to become concentrated along with the germanium at the silicon silicon-dioxide interface.